Most cases appear to have uncomplicated, typical influenza-like illness and recover spontaneously. To prove the effective of any protocol, drug/drugs should not be prescribed for first 36 hours but only Saline IV, to confirm that it is the body systems which recover on its course.

Fever has been absent in some outpatients and in up to 1 in 6 surviving hospitalized patients.

Gastrointestinal symptoms (nausea, vomiting and/or diarrhea) have occurred in up to 38% of outpatients in the United States.

Hospitalized patients. Approximately 2–5% of confirmed cases in the United States and Canada, as well as 6% in Mexico, have been admitted to hospital.

Among patients presenting with acute respiratory illness for care in Mexico, 13% tested positive for new influenza A (H1N1) virus infection (about one-fifth have had seasonal influenza), of whom about 10% have been hospitalized and one-third of those hospitalized required mechanical ventilation.

Almost one-half of the patients hospitalized in the United States, and 21 of 45 (46%) fatal cases in Mexico for whom data are available, have had underlying conditions, including pregnancy, asthma, other lung diseases, diabetes, morbid obesity, autoimmune disorders and associated immunosuppressive therapies, neurological disorders and cardiovascular disease. These conditions were initially due to cellular dehydration.

Among 20 pregnant women in the United States confirmed to have been infected with new influenza A (H1N1) virus, 3 required hospitalization, 1 of whom died; this patient had started antiviral therapy 13 days after illness onset.

Among 30 patients hospitalized in California, 64% had underlying conditions and 2 of 5 pregnant women developed complications, including spontaneous abortion and premature rupture of membranes.

Among 45 fatal cases in Mexico, 54% were among previously healthy people, most of whom were aged 20–59 years; 1 was a pregnant woman at 34 weeks’ gestation.

Case fatality ratios were lower in children and teenagers than in adults, for reasons to be determined. Rapidly progressive respiratory disease has accounted for most severe or fatal cases.

In Mexico, the median time from onset of illness to hospitalization was 6 days (range, 1–20 days) in 45 fatal cases, compared with a median of 4 days in hospitalized cases in the United States.

Several patients experienced cardiopulmonary arrest shortly after arrival at hospital.

Diarrhoea has been uncommon in hospitalized cases.

In Mexico, the clinical course has been notable for severe pneumonia, multifocal infiltrates including nodular alveolar and, less frequently, basilar opacities on chest radiographs, as well as rapid progression to acute respiratory distress syndrome (ARDS) and renal or multi-organ failure (24% of fatal cases).

The median time from symptom onset to death was 10 days (range, 2–33 days).

Of those hospitalized in California, 15 of 25 (60%) tested had radiographic changes suggestive of pneumonia, including 10 with multilobar infiltrates; 4 (13%) required mechanical ventilation.

Both leukocytosis and leucopenia have been found in those hospitalized.

Up to one-half of hospitalized patients have shown some degree of renal insufficiency, perhaps secondary to rhabdomyolysis and myoglobinuria, although other causes including hypotension, dehydration and hypoxia may be contributory.

Acute myocarditis has been suspected in some patients, but encephalitis has not been described to date.Microbiology and pathogenesis

Few patients have had evidence of bacterial infection upon admission, but instances of empyema, necrotizing pneumonia and bacterial coinfection, as well as ventilator-associated pneumonias, have occurred.

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